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Do you mean it has been dissolving faster, or dissipating faster? I think you mean that the FC level is going down faster, but I wasn't sure.......
I wish I could get readings as good as yours. I cant get my pH down past 7.8 for the life of me. Seems your water is almost stable. I thought Alk was suppose to be 125?Neo
I assume that you are using the same dichlor, so there is not a difference in the active % chlorine? And no change in your water source- no refill, right? The only thing that ever comes up here that would burn up your chlorine faster is the presence of "nasties." Perhaps take your filter and housing parts out, rinse, take all the headrests out, rinse, and make sure there is not some little nidus of badness hidden somewhere.
As your water ages, the build-up of CYA and TDS tends to interfere with santizer effectiveness. I find that I have to increase dosage if I keep the water after signs of TDS (fizzy water, takes longer to clear after jets are shut off) becomes evident. Anne's suggestion to look for hidden organics is a good one. Even though the filter rinses clean, there can be a bunch of junk in the fabric. I am always surprised how much stuff comes out when I chemically clean a filter that is white on the surface. I'm sure the chlorine tries to oxidize that material.
...It's been 9 weeks now since my initial fill and last week I raised my alk from 80 to 100 and my ph from 7.2 to 7.5...
A couple more thoughts for you: If I remember correctly, chlorine is 50% hypochlorous acid at pH 7.5 and 75% at 7.0, (roughly 5%/pt of pH) so the pH rise could account for 15% decrease in sanitizer efficiency. Since this is your initial fill, I assume you used the tub a lot in the first few weeks. You might be at the end of your water's life, even though you haven't made it to the magic 3 month mark. The Taylor book has a formula for estimating when it is time to change. It is based on frequency of use, so after the "new toy" feeling wears off, you will find that your water can stretch a lot longer. We rarely use ours in the summer, and I didn't need to change water for 5 months.